First of all, the spices really won't do anything. The next time you need to do a soil change, add a large percentage of rinsed perite. In the meantime, you could apply a layer of horticultural grade diatomaceous earth to the surface. It will prevent the adult (airborne) gnats from laying eggs and will also control the fungus gnat larvae in the soil. Continue with the gnatrol. If the larvae are controlled, new adults will cease being born.

DE can be found in in the organic gardening section of your garden center. It is not the same as pool filter DE.

Fungus gnat larva feed on the decaying vegetative material in your containers, although they need a fairly moist environment to exist. Yellow Sticky traps can be used to trap the adults. As the link about Fungus Gnats from the University of California will tell you allowing the surface of the soil to dry out is one control method. Since these wee buggers are more of a nuisance then anything else much stronger control methods are not too desirable.

Here is a link that might be useful: About Fungus Gnats

The Miracle Grow potting soils were notorious for this back in the day. But that was when used for indoor plants. So why would this happen outdoors ?
I'm sure cheaper big box store soils would have similar problems.

To the OP.
Somethings in life are just not worth the effort IMHO. I would bag and seal the gnat infested planting in it's entirety and toss it in the garbage. Take the soil you have left back to the store and get a refund. Purchase potting soil that has been pasteurized ? ( heated properly) and just give yourself a nice, clean fresh start. After all, your eating this stuff raw. All your out is the cost of some seeds, which is less that purchasing potential cures.

Looking forward to replies as to why this has happened outdoors. That's the new part to me. Hope to avoid it myself.

Plaid, since these are outdoor containers, we cannot presume that the gnats came from 'contaminated " soil. Potting media of most kinds are peat based....something that our native gnats will gravitate to like a moth to a porch light. It doesn't even have to be the crappy stuff.

You can upen a bag of sterile potting medium, pot up a brand new plant in a shiny new container, place it on your back patio and if it's on the peaty, dense side fungus gnats will find it in a heartbeat.

Fungus gnats are ubiquitous outside...just not very noticeable since they're....umm....outside.

The link I provided above also discusses Fungus Gnats in containers. if one reads what is there.
Yellow sticky traps are one part of a control program not the end all. If you trap some adults on those cards that means they will not be mating and laying eggs.

Yes, I did skim over that part....where it mentions that the larvae feed on roots and can be more than a nuisance. :-) Exactly.

Thanks all, I am gonna try the yellow sticky traps next... hope they can help.

Fungus gnats will not habitate in vermiculite. Next time anyone is using a shallow tray, use pure vermiculite as soil medium. Saves seedlings and great for mesclun.

Ive tried everything inexpensive. The yellow sticky traps did not work, although over my seed planted containers hardware cloth sprayed with adhesive did work a bit. But once the plants reach the mesh it must be removed.

Diatomaceous earth does work once the seedlings are thinned and you can spread it around the plants, but when they are growing thickly over the soil surface you cannot get the DE between them...but the gnats CAN get in there.

Ive sprayed bug repellant around the sides of thd pots...useless.

Ive trued the herbs and also essential oils. No go.

Diatomaceuous earth is the only thing that works ...once the plants have been thinned.